Differential exhumation of cratonic and non-cratonic lithosphere revealed by apatite fission-track thermochronology along the edge of the São Francisco craton, eastern Brazil

Lithosphere of cratons and orogens generally reacts differently to tectonic events. Although these differences are mostly clear during the orogenic phases, understanding how they respond to tectonic reactivation is still challenging. Here, we report the first detailed apatite fission-track (AFT) study pinpointing the gradual transition between cratonic and orogenic lithosphere, using the case study of the São Francisco craton (SFC) and the adjacent Araçuaí-West Congo Orogen (AWCO), eastern Brazil. The collision that built the AWCO partially affected the inherited rift structures of the Paramirim Aulacogen, embedded in the São Francisco-Congo paleocontinent. Our data reveal a differential Phanerozoic exhumation between closely interspaced areas affected and not affected by the AWCO deformation. Samples from the SFC present slow and protracted basement cooling during the Phanerozoic, while samples from the orogen display rapid exhumation since the Eocene. An intermediate ~ N–S zone of c.40 km shows lower magnitude basement cooling during the Cenozoic, possibly because the propagation of AWCO deformation decreases towards the craton interior. Within the orogen, the Rio Pardo salient is the main reactive structure and probably results from the deformation of a master fault, inherited from its precursor rift. Here, we show how the magnitude of Phanerozoic denudation may be deeply associated with previous events of lithosphere weakening.

www.nature.com/scientificreports/ Ediacaran-Cambrian orogeny 6,7 . The objective of our study is to gain deeper insights into the exhumation of the basement in an area of transition between the SFC and the AB, during subsequent Phanerozoic tectonic events and, thus, verify how different types of lithosphere exert control on later basement exhumation.

Geological background
The São Francisco craton (SFC) consists of an Archean-Paleoproterozoic basement older than 1.8 Ga, with Mesoproterozoic to Cambrian cover units 8 (Fig. 2). To the southeast of the craton, the Araçuaí belt (AB) involving a basement older than 1.8 Ga and Proterozoic-Cambrian cover complexes, encompasses two large scale features: (i) a large arcuate salient, the Rio Pardo Salient, and (ii) a NNW-trending so-called interference zone (IZ in the Fig. 1) with a subparallel oriented Proterozoic intracontinental rift structure, the Paramirim Aulacogen 8 (Figs. 1 and 2). The interference zone is characterized by the inversion of the normal faults of the aulacogen phase and nucleation of thrusts, folds, and dextral shear zones. The AB, as a whole, is a segment of the wider Araçuaí-West Congo Orogen (AWCO) (Fig. 1), which developed along the margin of the SFC and the Congo Craton during the Ediacaran to Early Cambrian ( 9 and references therein). The AWCO evolved due to the closure of an embayment, i.e. the terminal branch of the Adamastor ocean, carved into the São Francisco-Congo paleocontinent, and eventually resulted in the assembly of West Gondwana 9 .  The SFC and the AB remained confined in the interior of West Gondwana from the early Paleozoic until the rifting and break-up of this supercontinent around 130 Ma 5 . The separation between South America and Africa took place with the opening of the South Atlantic Ocean and resulted in the shifting of the regional base level of our study area towards the present-day coast. Currently, the Contas River drains the area to the east, while the São Francisco River drains it to the north (Fig. 3). Erosion-resistant quartzite ridges from the Espinhaço Supergroup (Figs. 2 and 3) form the N-S oriented watershed that reaches up to 1400 m altitude. Diffuse and thin (meters thick) Cenozoic deposits are present in the main river channels and laterized plateaus, mainly in the Conquista Plateau (Fig. 3). This plateau is also relatively elevated to around 1000 m and partially overlaps the Rio Pardo salient structures, where it delimits the the Contas River Basin to the south.
Apatite fission-track thermochronology: method and results. Thirty-two basement samples (gneiss, granites and migmatites) were collected from the basement of the AB-SFC boundary (Fig. 2) to perform low-temperature thermochronology with the AFT method. In order to explore the effects of northwards decreasing deformation, we collected closely interspaced samples in an N-S transect, crossing the limit orogen/ craton. Another transect was sampled in the Rio Pardo salient structure. Sample lithology, locations, and elevations are found in Table 1.
The AFT method is a low-temperature dating technique based on the accumulation of mineral lattice damage, i.e. 'fission tracks' , generated by the spontaneous fission of 238 U 11 . These tracks are preserved in the apatite lattice on geological time scales at temperatures lower than c.120 °C, i.e. upper crustal temperatures 12 . AFT ages are hence cooling ages registering the time since the fission tracks became thermally stable in the apatite crystals. After etching to reveal the natural or spontaneous tracks by optical microscopy, the observed track density per unit area is a measure for the AFT age 11 . At temperatures between c.120-60 °C, fission tracks are able to accumulate in the apatite lattice but are subject to track length shortening due to thermal annealing or lattice restoration. Hence, a track length distribution of a sample is an indicator of the thermal history experienced by the apatite and its host rock 12,13 .
Apatite grains were concentrated using standard procedures, hand-picked and embedded in epoxy resin 14 . Mounts with c.120 apatite grains were etched for 20 s in 5.5 M HNO 3 solution at 21 °C to reveal spontaneous fission tracks 15 . In this study we applied the external detector (ED) approach using thermal neutron irradiation 16 . U-free mica (Goodfellow, clear ruby) was attached as ED on top of each sample and age standard (Durango and Fish Canyon Tuff) mount 17 . IRMM-540 dosimeter glasses were used for monitoring the thermal neutron fluence. The packages were irradiated at the Belgian Nuclear Research Centre (SCK, Mol) using the Belgian Reactor 1 (BR1) facility (Channel X26; 18 ). After irradiation, the ED was etched using 40% HF for 40 min at 21 °C in order to reveal induced fission tracks.
For each sample, 20 or more apatite grains were analysed. Fission track density was measured using a motorized Nikon Eclipse Ni-E microscope with a DS-Ri2 camera attached, at a 1000 × magnification. Central age calculation was performed using "IsoplotR" 19 with an overall weighted mean zeta value of 330.6 ± 3.9 a·cm 2 (Analyst AF) based on multiple Durango and Fish Canyon Tuff apatite age standards and the IRMM-540 dosimeter www.nature.com/scientificreports/ glass 20 . All samples pass the χ 2 test, indicating single age populations. In 23 samples, where it was possible to achieve a representative AFT length-frequency histogram (n > 50), Markov Chain Monte Carlo (MCMC) inverse modeling was performed using the QTQt software 10 . Except for the present-day temperature (25 ± 15 °C), no time-temperature constraints were added. The Ketcham annealing model 21 , with D par as the kinetic parameter was used. The data is summarized in Table 1.
Despite the long-wavelength topography between 515 and 816 m elevation, and the low difference of erodibility of the sampled rocks (gneiss/migmatites), the AFT central ages vary substantially between 102 ± 14 Ma and 392 ± 21 Ma (Fig. 2), showing no correlation with D par (see Supplementary S.7 online). The mean track length (MTL) values are short to intermediate (9.9 to 12.8 µm) with mainly unimodal distributions (see Supplementary S.1 online), indicating long residence in the apatite partial annealing zone (APAZ, c.120-60 °C). Track lengths and their angle with the c-axis show no evident correlation (see Supplementary S.8 and S.9 online). Neither is this the case for the MTL vs. AFT central age plot (Fig. 4). From the thermal history modelling, three main t-T trends can be distinguished (Fig. 2

Discussion
The AFT dataset distinctively reveals differential basement cooling and hence exhumation patterns. Samples from the northernmost area, close to Riacho de Santana city (TJ 21-26; models i in Fig. 2), do not show any important cooling during the Meso-Cenozoic. According to the models, slow and protracted cooling through the APAZ brought these samples (TJ [21][22][23][24][25][26] close to the surface (< 60 °C) even before the Jurassic, indicating that Cretaceous Atlantic rift and post-rift thermal events are lacking or exerted limited influence in this area. These samples (TJ 21-26) are from a previously identified cratonic segment of the Paramirim Aulacogen 22 , confirming no or very low degrees of tectonic rejuvenation of this lithospheric segment. The results are also in accordance with previous data from the southwestern SFC (Fig. 1), where AFT ages are older than the Jurassic 4 , and data from cratonic areas close to the Atlantic margin, where several samples are also older and do not seem affected by Cretaceous rifting or posterior events 23 (Fig. 1). Areas with no clear signal in the AFT system from the rifting phase have indeed been associated to cratonic rheology (e.g. 4,24,25 ). The rifting architecture varies greatly depending on the crustal thickness, the structural fabric and directions, the lithospheric rheology, and potential magmatic activity (e.g. [26][27][28]. With respect to our study region, we can conclude that the cratonic region of the Paramirim Aulacogen inherits the rigidity from the SFC. In this case, epirogenic uplift during the opening of the South Atlantic was mitigated as well as the erosional response to this process, resulting in an almost stable thermal structure. Contrary to the craton, our data in the AB indicate relatively fast exhumation during the Eocene-present (models ii and iii in Fig. 2), with AFT ages not older than the Jurassic. Hence, confirming that our thermochronology data present a good correlation between the delimitation of the SFC and the AB. During the Cenozoic, the study area was embedded in the relatively stable South America platform, far from plate boundaries. The observed basement cooling must most probably be the consequence of erosional exhumation as indicated by the sediment supply to the adjacent basins. In the Eocene, the Jequitinhonha, Almada and Camamu passive margin basins, which are connected by river systems to our study area (Fig. 3), started their regressive phase. Progradation wedges, made up of coarse-grained sandstones, platform carbonates and distal mudstones, accumulated until the recent and reach 500 m in thickness [29][30][31][32] . Onshore, forming the typical regional tableland geomorphology, siliciclastic continental-to-shallow-water marine sediments of the Barreiras Formation were deposited from the Upper Oligocene onwards 33 . Thereby, we suggest that during the Eocene to present, our study area was heterogeneously eroded, likely partially contributing as a source of sediments to the above-mentioned deposits.
Previously published thermochronological data also indicates that the AB was indeed deeply exhumed during the Cenozoic (e.g. 4,14,23 ) and, thus, our data further underscores the tracing of the belt (Fig. 2). Additionally, the Rio Pardo Salient seems one of the main structures in concentrating deformation since samples from this domain (TJ 11-16 and TJ 30-34) exhibit c.50 °C of rapid cooling during the Eocene to recent (models ii in Fig. 2). This salient presents a relatively high topography (c. 1000 m) in the Conquista Plateau region (Fig. 3), which may be a product of the reactivations from the Eocene. The formation of the Rio Pardo Salient resulted from the closure of the depocenter of the Neoproterozoic Macaúbas Basin, precursor to the AWCO collision 34 . The structure prior to the salient deformation was probably a major fault of the Macaúbas basin, deeply rooted through the crust. Analogue modelling has demonstrated that master faults are, indeed, more likely to concentrate stress and localize vertical displacements under compressional stress fields 35 . It is also important to note that the Rio Pardo Salient area (where the TJ 30-34 samples are located) limits the outcrops of the Macaúbas Basin fill (Fig. 2), supporting the location of the master fault.
Other authors also identified Neogene denudation pulses in northeastern Brazil using low-temperature thermochronology 36,37 , indicating that the event was widespread. Although climate has already been proposed as a main driving force for intensifying erosional denudation and hence basement cooling 37 , our results indicate that the denudation was far more heterogeneous. Evidence of brittle tectonics, related with ongoing ENE-WSWoriented compression 38 , has been identified within the Cenozoic deposits (e.g. 39 ) and challenged the concept of relative tectonic inactivity of passive margins. In line with the ideas of 36 , far-field partitioning of contemporaneous intraplate stress from the Andean collision zone seems to be the most plausible driving mechanism for the compressional tectonic reactivation of the intracontinental region, mainly during the Incaic and Quechua phases (Fig. 4). This final rapid cooling event most probably (partially) erased evidence from previous thermal events, Ma. This latter event can however still be observed in three of our thermal history models (TJ [14][15][16]. Although in some areas it is easy to distinguish craton vs. orogen behavior, the transition between them is not always clearly traceable. In general, stress and resulting deformation decrease from the collisional zone to the plate's interior. In our data, we could identify an intermediate t-T path (models iii in Sect. 3; Fig. 2) with c.35 °C of Eocene-present cooling (from 60 °C to surface temperature). In this way, it is possible to use the decrease of recent basement cooling as documented by AFT data, to trace the transition from orogenic to cratonic lithosphere. In the main N-S transect (Fig. 2), this intermediate zone is about 40 km in S-N direction and was probably affected by AWCO orogeny but with less penetrative deformation. In the area where samples TJ 17-20 were collected, close to Guanambi city, a set of dextral reverse shear zones 40 seems to be related to this intermediate zone and then part of the orogen area. Based on our observations, we propose an adjustment of the AB boundaries within the Paramirim Aulacogen (Fig. 2). We also suggest the compartmentalization of the orogen (Fig. 5) in weak zones (e.g. Rio Pardo Salient) and the area of decreasing rigidity (i.e. transitional zone) until finally the craton s.s. is reached. Remarkably, the heterogeneous basement cooling patterns in our study area, likely resulting from the same stress regime, reinforces the concept that some lithospheric segments are more easily deformed than others.

Conclusions
AFT data from the São Francisco craton and adjoining Araçuaí belt in the Paramirim Aulacogen area (eastern Brazil) elucidate the differential behavior of the cratonic and non-cratonic lithosphere during the Phanerozoic exhumation of this region. To the north of our study area, thermal history modelling of the basement rocks exhibits slow and protracted cooling during the Phanerozoic, consistent with the rigid cratonic lithosphere of the São Francisco Craton. Samples from the Araçuaí belt, in the interior of the Paramirim aulacogen, display reactivation during the Cenozoic, mainly between the Eocene to present, reflecting its weakened lithosphere, inherited from the Ediacaran-Cambrian collision. An intermediate zone is identified, and it is considered mostly part of the Araçuaí Belt but with less penetrative deformation as to the orogen proper. The thermochronological data